Sunday, February 28, 2016

Not Without Warning - The Return Of Mosquito Disease Threats

Long before Zika arrived in the Americas (in 2014), before Chikungunya jumped to the Caribbean (in 2013), and even before Dengue re-emerged after 70 years in Key West (in 2009), researchers have been warning us that the Americas and Europe are not safe from emerging or re-emerging mosquito disease threats.

The maps below show that diseases like Malaria and Yellow Fever once plagued much of the settled United States (with epidemics as far north as Boston and Michigan), until comprehensive mosquito control efforts during the early part the 20th century successfully beat them back.

The last major Yellow Fever outbreak in the United States was in New Orleans in 1905, but it wasn't until 1949 that the U.S. was declared free of malaria as a significant public health problem (cite).

While there are societal factors (mosquito control, window screens, air conditioning, etc.) that will presumably help limit any outbreak's impact, we have both the climate and the right mosquito vectors to support a variety of mosquito borne illnesses, including Yellow Fever, Dengue, Malaria, Chikungunya and Zika.

And we need look only to the arrival of West Nile Virus in 1999 - a mosquito-borne illness that prior to the mid-1990s was considered mostly mild, but which has since proved to cause significant neuroinvasive disease in a minority of cases (sound familiar?) - to see how quickly a new threat can spread.

West Nile virus (WNV) has quickly established itself in North America
since its recognition in New York City in 1999. Historically, WNV has
been associated with temporally dispersed outbreaks of mild febrile
illness. In recent years, the epidemiology and clinical features of the
virus appear to have changed, with more frequent outbreaks associated
with more severe illness being noted. The 2002 outbreak in North America
was unprecedented in terms of the number of cases and geographic spread
of the virus. Historical patterns of WNV provide few indications as to
the future behavior of WNV in North America.

(SNIP)

WEST NILE VIRUS SINCE 1996

Beginning
around 1996, the epidemiology and clinical spectrum of WNV appeared to
change. A large outbreak of WNV occurred in the area around Bucharest,
Romania, and was notable for a number of reasons (14, 15).
It was the first WNV outbreak to be centered in a predominantly urban
area, and it was the first outbreak of the virus in which the
preponderance of symptomatic cases involved CNS infection.

Since WNV arrived in NYC, it has spread to all 50 states, and causes tens of thousands of mild West Nile Fever cases every year, and anywhere from a few hundred to several thousand cases of severe neuroinvasive West Nile Virus.

WNV has an advantage in that it has a sylvatic cycle (birds to mosquitoes). Human infection is basically a result of incidental collateral damage (see graphic below).

Zika, Dengue, and Chikungunya - at least outside of Africa and Asia -
have no known non-human animal reservoirs, which means they have a
harder time becoming endemic. But when enough people become infected,
these viruses are able to sustain themselves in an Urban
Cycle, where transmission is strictly human-to-mosquito-to human.

In the middle of the last decade we saw Chikungunya made a break from Africa, and jump to Reunion Island in the Indian Ocean where it reportedly infected about 1/3rd of that island’s population (266,000 case out of pop.770,000) in a matter of a few months, before moving on to Southeast Asia.

About the same time, Dengue began to turn up again in the United States after decades of absence. In January of 2009, in Outnumbered By A Competent Vector, we looked at reports of Dengue's incursions into Texas and Queensland, Australia.

Meanwhile, this growing threat of mosquito-borne illnesses wasn't going unnoticed in Europe, and in 2010 the ECDC's journal Eurosurveillance devoted a special edition to the threat to the EU from Vector-borne diseases - December 2010.

Also in 2010, we saw an unusually severe Dengue outbreak in Puerto Rico, that before it was quashed, had infected more than 21,000 people, killing 31 (see MMWR: Dengue Epidemic In Puerto Rico). Globally, Dengue has seen a 30-fold increase over the past 50 years (see chart below).

The
Pan American Health Organization/World Health Organization (PAHO/WHO),
in collaboration with the U.S. Centers for Disease Control and
Prevention (CDC), has published new guidelines on chikungunya, a
mosquito-transmitted virus transmitted that causes fever and severe
joint pain. The Guidelines for Preparedness and Response for
Chikungunya Virus Introduction in the Americas aims to help countries
throughout the Americas improve their ability to detect the virus and be
prepared to monitor, prevent, and control the disease, should it
appear.

Mosquito borne diseases sicken and kill millions of people each year, and we've had decades of warning thatthey could someday return to the United States (or Europe), but the threat has remained largely ignored by the public.

Regardless how Zika pans out, it isn't the only current threat (i.e. Dengue, CHKV, Malaria, etc.), nor is it likely to be the last obscure mosquito-borne virus that will emerge from the jungles and threaten global health. Nature has a very deep bench.